The "cluster" package has a Monte Carlo option. "R2WinBUGS" is the standard Monte Carlo package. This info page lists many options. I've used the latter. http://cran.r-project.org/web/views/Bayesian.html
My new MonteCarlo package can help you with that. It also comes with a vignette that contains some simple examples so you can learn how to use it very quickly.